>
>
> On Tue, Jan 7, 2014 at 11:14 AM, John Clark <johnkcl...@gmail.com> wrote:
>
>> On Mon, Jan 6, 2014 at 2:36 PM, meekerdb <meeke...@verizon.net> wrote:
>>
>> > he assumed this time asymmetry was fundamental, not a mere statistical
>>> effect related to the low entropy of the initial conditions of the
>>> experiment.
>>>
>>
>> A mere statistical effect?? I would argue that the second law of
>> thermodynamics is much more fundamental than the first. The first law, the
>> idea that matter and energy can not be created or destroyed is not a
>> logical necessity it's merely a empirical observation, up to now we've just
>> never seen that law violated and we use induction to conclude that we never
>> will. Induction is a very good rule of thumb but it you wait long enough it
>> can sometimes lead you astray. I don't expect it to happen but I can at
>> least conceive of the idea that someday we will find a circumstance where
>> the first law is untrue.
>>
>> But the second law is not like that, conceiving of a world where entropy
>> doesn't increase with time is like imagining what the world would be like
>> if 2+2=5. The second law is not based on observation but on pure logic and
>> the fact that there are just more ways to be disorganized than organized.
>> Science always changes but if I had to pick one thing that would still be
>> valid in a thousand or even a million years it would be the second law.
>>
>
> Are you disputing the "statistical effect" part, or the "mere" part?
> Because while it's true that the second law can be derived theoretically
> from some fairly broad assumptions, this derivation is itself a statistical
> one which assigns a nonzero probability to spontaneous decreases in
> entropy. Also, I think it's going too far to say that imagining a world
> where entropy doesn't increase is like imagining a world where 2+2=5, since
> you do need *some* assumptions about the laws of physics to derive the
> second law. For example, in a world where volume in phase space wasn't
> conserved by the dynamics (Liouville's theorem in classical statistical
> mechanics), so a collection of initial states occupying a large region of
> phase space could all converge on a smaller number of later states
> occupying a smaller region of phase space (meaning that information can be
> lost, since multiple initial states can lead to the same later state
> leaving you with no way to determine what the initial state was), you could
> well have consistent decreases in entropy with time.
>
> Even with the laws of physics we know, if you don't assume the universe
> *starts* in a state of low entropy, then the 2nd law should actually be
> time-symmetric, given that the fundamental laws of physics themselves are
> time-symmetric (or CPT-symmetric in quantum field theory). With
> time-symmetric laws, if you are given the state of an isolated system at
> some time t, the laws which allow you to predict the future state later
> than t can just as easily be used to "retrodict" the past state at times
> earlier than t. So if you at some time t you find a system in a macrostate
> of lower-than-maximum entropy, and you retrodict its past in this way,
> exactly the same statistical reasoning tells you its entropy is far more
> likely to have been higher in the past of t, rather than lower as we might
> intuitively expect. This may seem strange, but to see why it makes physical
> sense, imagine you have an isolated system (a box of gas, say) that is
> known to have reached maximum entropy at some time in the past. Then if you
> choose a random time to observe it and find the entropy to be
> lower-than-maximum, it actually is most likely that you are looking at the
> local minima of a random entropy-decreasing statistical fluctuation, which
> means that entropy should be symmetrical higher both shortly before and
> shortly after the observation time.
>
> So, to explain the asymmetry between past and future--the "arrow of
> time"--the 2nd law alone won't do it. You need to either assume low-entropy
> conditions at the Big Bang (which might themselves have some explanation in
> an "eternal inflation" theory, see for example Sean Carroll's theory which
> he summarizes in a blog post at
> http://preposterousuniverse.blogspot.com/2004/10/arrow-of-time.html and
> also in the paper linked to there, and further in his book "From Eternity
> to Here"), or some basic time-asymmetry in the fundamental laws of physics,
> although I get the sense that the second is not a very popular approach to
> explaining the arrow of time among physicists (and it's not really clear to
> me what the connection would be between time-asymmetry in some new theory
> like quantum gravity, and the failure of the 2nd law to apply in reverse).
>
Jesse,
That is interesting. Is it the same idea that David Layzer uses here
http://www.informationphilosopher.com/problems/arrow_of_time/ to use the
expansion of the universe to explain the increasing room for explaining how
the room for possible entropy grows faster than energy and matter
equilibration?
Jason
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